U.S. patent number 10,121,554 [Application Number 15/846,766] was granted by the patent office on 2018-11-06 for multi-gate nor flash thin-film transistor strings arranged in stacked horizontal active strips with vertical control gates.
This patent grant is currently assigned to SUNRISE MEMORY CORPORATION. The grantee listed for this patent is Sunrise Memory Corporation. Invention is credited to Eli Harari.
United States Patent |
10,121,554 |
Harari |
November 6, 2018 |
Multi-gate nor flash thin-film transistor strings arranged in
stacked horizontal active strips with vertical control gates
Abstract
Multi-gate NOR flash thin-film transistor (TFT) string arrays
("multi-gate NOR string arrays") are organized as stacks of
horizontal active strips running parallel to the surface of a
silicon substrate, with the TFTs in each stack being controlled by
vertical local word-lines provided along one or both sidewalls of
the stack of active strips. Each active strip includes at least a
channel layer formed between two shared source or drain layers.
Data storage in the TFTs of an active strip is provided by
charge-storage elements provided between the active strip and the
control gates provided by the adjacent local word-lines. Each
active strip may provide TFTs that belong to one or two NOR
strings, depending on whether one or both sides of the active strip
are used.
Inventors: |
Harari; Eli (Saratoga, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sunrise Memory Corporation |
Los Gatos |
CA |
US |
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Assignee: |
SUNRISE MEMORY CORPORATION (Los
Gatos, CA)
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Family
ID: |
58406670 |
Appl.
No.: |
15/846,766 |
Filed: |
December 19, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180108423 A1 |
Apr 19, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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15220375 |
Jul 26, 2016 |
9892800 |
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62235322 |
Sep 30, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H05K
999/99 (20130101); G11C 16/26 (20130101); H01L
27/11582 (20130101); G11C 16/0483 (20130101); G11C
16/28 (20130101); G11C 11/5635 (20130101); G11C
16/3427 (20130101); G11C 16/3431 (20130101); H01L
27/0688 (20130101); H01L 29/7926 (20130101); G11C
16/10 (20130101); G11C 16/0491 (20130101); H01L
29/40117 (20190801); H01L 29/66833 (20130101); H01L
27/11573 (20130101); G11C 11/5628 (20130101); G11C
16/0416 (20130101); G11C 16/0466 (20130101); H01L
27/11565 (20130101) |
Current International
Class: |
G11C
16/34 (20060101); G11C 11/56 (20060101); H01L
29/66 (20060101); G11C 16/04 (20060101); H01L
29/792 (20060101); H01L 27/06 (20060101); H01L
21/28 (20060101); G11C 16/28 (20060101); G11C
16/26 (20060101); G11C 16/10 (20060101); H01L
27/11582 (20170101); H01L 27/11573 (20170101); H01L
27/11565 (20170101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report and Written Opinion of PCT Application
No. PCT/US2016/044336 dated Dec. 12, 2016. cited by applicant .
Lue, HT et al., "A Highly Scalable 8-Layer 30 Vertical-Gate (VG)
TFT NANO Flash Using Junction-Free Buried Channel BE-SONOS Device",
published in the 2010 Symposium on VLSI Technology Digest of
Technical Papers, pp. 131-132. cited by applicant .
Kim, W. et al., "Multi-Layered Vertical Gate NANO Flash Overcoming
Stacking Limit for Terabit Density Storage", published in the 2009
Symposium on VLSI Technology Digest of Technical Papers, pp.
188-189. cited by applicant .
Wann, H.C., "High-Endurance Ultra-Thin Tunnel Oxide in Monos Device
Structure for Dynamic Memory Application", published in IEEE
Electron Device letters, vol. 16, No. 11, Nov. 1995, pp. 491-493.
cited by applicant.
|
Primary Examiner: Dinh; Son
Assistant Examiner: Leboeuf; Jerome
Attorney, Agent or Firm: Kwok, Esq.; Edward C. VLP Law
Group, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is a divisional of U.S. patent application
Ser. No. 15/220,375, filed Jul. 26, 2016, entitled "Multi-Gate NOR
Flash Thin-Film Transistor Strings Arranged In Stacked Horizontal
Active Strips With Vertical Control Gates," which relates to and
claims priority of U.S. provisional patent application Ser. No.
62/235,322, filed Sep. 30, 2015, entitled "Multi-Gate NOR Flash
Thin-Film Transistor Strings Arranged In Stacked Horizontal Active
Strips With Vertical Control Gates." Both Applications are hereby
incorporated by reference in their entirety.
Claims
The invention claimed is:
1. A NOR string array formed over a planar surface of a
semiconductor substrate, comprising a first NOR string and a second
NOR string, each NOR string comprising thin-film storage
transistors formed along an active strip extending lengthwise along
a first direction parallel to the planar surface, wherein the thin
film storage transistors in each NOR string share a pre-charge
device, a common drain terminal and a common source terminal,
wherein a gate terminal of each storage transistor in the first NOR
string is electrically connected to a gate terminal of a
corresponding storage transistor in the second NOR string by a
conductor that extends lengthwise along a second direction which is
substantially perpendicular to the planar surface, and wherein the
pre-charge device pre-charges the common source terminal to a
voltage that is substantially held by virtue of a parasitic
capacitance in the source terminal during a program,
program-inhibit, reading or erasing operation on the NOR string,
further comprising a third NOR string, wherein the first NOR string
and the third NOR string are formed on a plane parallel the planar
surface, wherein one or more storage transistors on the third NOR
string are programmed to have reference threshold voltages, which
are compared to threshold voltages of storage transistors in the
first NOR string during programming or read operations.
2. The NOR string array of claim 1, wherein the active strip of the
first NOR string and the active strip of the second NOR string are
both formed on a plane parallel to the planar surface.
3. The NOR string array of claim 1, wherein the third NOR string
provides a reference string leakage current source by setting all
the storage transistors of the third NOR string are in an off
state.
4. The NOR string array of claim 1, further comprising a plurality
of reference NOR strings, wherein when the storage transistors of
the first NOR string are programmed according to a multibit scheme,
the reference NOR strings each provide a reference for a different
state of the multibit scheme.
5. The NOR string array of claim 1, wherein the conductor comprises
one of N.sup.+ doped polysilicon, P.sup.+ doped polysilicon, and a
refractory metal of a high work function with respect to silicon
dioxide.
6. The NOR string array of claim 1, wherein the common drain
terminal comprises a semiconductor layer and a metallic layer that
is in electrical contact with, and in substantial alignment
lengthwise with, the semiconductor layer.
7. The NOR string array of claim 1, wherein each active strip has a
first side-edge and a second side-edge on opposite sides of the
active strip, wherein an independently controlled NOR string is
formed along each side-edge.
8. The NOR string array of claim 1, further comprising a first
global wiring which is in electrical contact with the conductor and
which extends lengthwise along a third direction that is
substantially orthogonal to both the first and second
direction.
9. The NOR string array of claim 8, wherein support circuitry for
memory operations is formed on or in the semiconductor substrate
and wherein the first global wiring conducts electrical signals
from the support circuitry to the conductor.
10. The NOR string array of claim 9, wherein the support circuitry
comprises voltage sources for providing the configurations of
voltages to be imposed on the terminals of each storage
transistors, so as to effectuate program charging, program-inhibit
charging, reading or erasing data stored in the storage
transistor.
11. The NOR string array of claim 8, wherein the first global
wiring is formed between the semiconductor substrate and the active
strip of the first NOR string.
12. The NOR string array of claim 8, wherein the NOR string array
further comprises a second global wiring formed above the active
strip of the first NOR string, the first and second global wirings
are in electrical contact with gate terminals of different storage
transistors of the first NOR string.
13. The NOR string array of claim 1, wherein the pre-charge device
equalizes the voltages of the common source terminal and the common
drain terminal.
14. The NOR string array of claim 12, wherein the common drain
terminal is charged to one of several predetermined voltages prior
to turning on the pre-charge device, the predetermined voltage
being selected from one of: program, program-inhibit, reading and
erasing data of the storage transistors.
15. The NOR string array of claim 1, wherein the common source
terminal serves as a shared virtual ground reference and the common
drain terminal serves as a common bit line for the storage
transistors in each NOR string.
16. A NOR string array formed over a planar surface of a
semiconductor substrate, comprising a first NOR string and a second
NOR string, each NOR string comprising thin-film storage
transistors formed along an active strip extending lengthwise along
a first direction parallel to the planar surface, wherein the thin
film storage transistors in each NOR string share a pre-charge
device, a common drain terminal and a common source terminal,
wherein a gate terminal of each storage transistor in the first NOR
string is electrically connected to a gate terminal of a
corresponding storage transistor in the second NOR string by a
conductor that extends lengthwise along a second direction which is
substantially perpendicular to the planar surface, and wherein the
pre-charge device pre-charges the common source terminal to a
voltage that is substantially held by virtue of a parasitic
capacitance in the source terminal during a program,
program-inhibit, reading or erasing operation on the NOR string,
wherein the first NOR string and the second NOR string are formed
on first and second planes parallel to the planar surface,
respectively, the NOR string array further comprising one or more
reference NOR strings on each of the first and second planes, and
wherein for each of the reference NOR strings, one or more storage
transistors on the reference NOR string are programmed to have
reference threshold voltages, which are compared during programming
and read operations to threshold voltages of storage transistors in
the corresponding NOR string on the same plane as the reference NOR
string.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to high-density memory structures. In
particular, the present invention relates to high-density memory
structures formed by interconnected thin-film storage elements,
such as thin-film storage transistors.
2. Discussion of the Related Art
In this disclosure, memory circuit structures are described. These
structures may be fabricated on planar semiconductor substrates
(e.g., silicon wafers) using conventional fabrication processes. To
facilitate clarity in this description, the term "vertical" refers
to the direction perpendicular to the surface of a semiconductor
substrate, and the term "horizontal" refers to any direction that
is parallel to the surface of that semiconductor substrate.
A number of high-density non-volatile memory structures, sometimes
referred to as "three-dimensional vertical NAND strings," are known
in the prior art. Many of these high-density memory structures are
formed using thin-film storage transistors formed out of deposited
thin-films (e.g., polysilicon thin-films), and organized as arrays
of "memory strings." One type of memory strings is referred to as
NAND memory strings or simply "NAND strings". A NAND string
consists of a number of series-connected storage transistors
("TFTs"). Reading or programming any of the series-connected TFTs
requires activation of all series-connected TFTs in the NAND
string. Under this NAND arrangement, the activated TFTs that are
not read or programmed may experience undesirable program-disturb
or read-disturb conditions. Further, TFTs formed out of polysilicon
thin films have much lower channel mobility--and therefore higher
resistivity--than conventional transistors formed in a
single-crystal silicon substrate. The higher series resistance in
the NAND string limits the number of TFTs in a string in practice
to typically no more than 64 or 128 TFTs. The low read current that
is required to be conducted through a long NAND string results in a
long latency.
Another type of high density memory structures is referred to as
the NOR memory strings or "NOR strings." A NOR string includes a
number of storage transistors each connected to a shared source
region and a shared drain region. Thus, the transistors in a NOR
string are connected in parallel, so that a read current in a NOR
string is conducted over a much lesser resistance than the read
current through a NAND string. At the present time, the present
inventor is not aware of any NOR string in the prior art that is
formed out of TFTs. To read or program a storage transistor in a
NOR string, only that storage transistor needs to be activated
(i.e., "on" or conducting), all other storage transistors in the
NOR string remain dormant (i.e., "off" or non-conducting).
Consequently, a NOR string allows much faster sensing of the
activated storage transistor to be read and avoids program-disturb
or read-disturb conditions in the other storage transistors of the
NOR string that are not read or programmed.
Three-dimensional memory structures are disclosed, for example, in
U.S. Pat. No. 8,878,278 to Alsmeier et al. ("Alsmeier"), entitled
"Compact Three Dimensional Vertical NAND and Method of Making
Thereof," filed on Jan. 30, 2013 and issued on Nov. 4, 2014.
Alsmeier discloses various types of high-density NAND memory
structures, such as "terabit cell array transistor" (TCAT) NAND
arrays (FIG. 1A), "pipe-shaped bit-cost scalable" (P-BiCS) flash
memory (FIG. 1B) and a "vertical NAND" memory string structure.
Likewise, U.S. Pat. No. 7,005,350 to Walker et al. ("Walker I"),
entitled "Method for Fabricating Programmable Memory Array
Structures Incorporating Series-Connected Transistor Strings,"
filed on Dec. 31, 2002 and issued on Feb. 28, 2006, also discloses
a number of three-dimensional high-density NAND memory
structures.
U.S. Pat. No. 7,612,411 to Walker ("Walker II"), entitled
"Dual-Gate Device and Method" filed on Aug. 3, 2005 and issued on
Nov. 3, 2009, discloses a "dual gate" memory structure, in which a
common active region serves independently controlled storage
elements in two NAND strings formed on opposite sides of the common
active region.
U.S. Pat. No. 6,744,094 to Forbes ("Forbes"), entitled "Floating
Gate Transistor with Horizontal Gate Layers Stacked Next to
Vertical Body" filed on Aug. 24, 2001 and issued on Jun. 1, 2004,
discloses memory structures having vertical body transistors with
adjacent parallel horizontal gate layers.
U.S. Pat. No. 6,580,124 to Cleaves et al, entitled "Multigate
Semiconductor Device with Vertical Channel Current and Method of
Fabrication" filed on Aug. 14, 2000 and issued on Jun. 17, 2003,
discloses a multibit memory transistor with two or four charge
storage mediums formed along vertical surfaces of the
transistor.
A three-dimensional memory structure, including horizontal NAND
strings that are controlled by vertical polysilicon gates, is
disclosed in the article "Multi-layered Vertical gate NAND Flash
Overcoming Stacking Limit for Terabit Density Storage" ("Kim"), by
W. Kim at al., published in the 2009 Symposium on VLSI Tech. Dig.
Of Technical Papers, pp 188-189. Horizontal 3D NAND strings with
vertical poly gates. Another three-dimensional memory structure,
also including horizontal NAND strings with vertical polysilicon
gates, is disclosed in the article, "A Highly Scalable 8-Layer 3D
Vertical-gate (VG) TFT NAND Flash Using Junction-Free Buried
Channel BE-SONOS Device," by H. T. Lue et al., published in the
2010 Symposium on VLSI: Tech. Dig. Of Technical Papers,
pp.131-132.
In the memory structures discussed herein, stored information is
represented by the stored electric charge, which may be introduced
using any of a variety of techniques. For example, U.S. Pat. No.
5,768,192 to Eitan, entitled "Non-Volatile Semiconductor Memory
Cell Utilizing Asymmetrical Charge Trapping," filed on Jul. 23,
1996 and issued on Jun. 16, 1998, discloses NROM type memory
transistor operation based on a "hot electron channel injection"
technique. Other techniques include Fowler-Nordheim tunneling used
in TFT NAND strings, and direct tunneling, both of which are known
to those of ordinary skill in the art.
SUMMARY
According to one embodiment of the present invention, multi-gate
NOR flash thin-film transistor (TFT) string arrays ("multi-gate NOR
string arrays") are organized as stacks of horizontal active strips
running parallel to the surface of a silicon substrate, with the
TFTs in each stack being controlled by vertical local word-lines
provided along one or both sidewalls of the stack of active strips.
Each active strip includes at least a channel layer formed between
two shared source or drain layers. Data storage in the TFTs of an
active strip is provided by charge-storage elements provided
between the active strip and the control gates provided by the
adjacent local word-lines. Each active strip may provide TFTs that
belong to one or two NOR strings, depending on whether one or both
sides of the active strip are used.
In one embodiment, only one of the shared source or drain layers in
an active strip is connected to a supply voltage, while the other
source or drain layer is held at a voltage determined by a quantity
of charge deposited in the source or drain layer. Prior to a read,
write or erase operation, the TFTs that are not to be activated act
as a strip capacitor, with one plate being the source or drain
layer itself and the other plate being the control gate electrodes
in the NOR string that are referenced to a ground reference. The
charge on the strip capacitor is provided by one or more pre-charge
TFTs that are activated momentarily to transfer charge to the strip
capacitor from a supply voltage connected to the contacted source
or drain layer.
In one embodiment, TFTs are formed on both vertical side edges of
each active strip, so that vertical local word-lines may be
provided along both vertical side edges of the active strips. In
that embodiment, double-density is achieved by having the local
word-lines along one of vertical edges of an active strip be
contacted by horizontal global word-lines provided above the active
strip, while the local word-lines along the other vertical edge of
the active strip be contacted by horizontal global word-lines
provided beneath the active strip. All global word-lines may run in
a direction perpendicular to the direction of the corresponding
active strips. Even greater storage density may be achieved by
storing more than one bit of data in each TFT.
Organizing the TFTs into NOR strings--rather than the prior art
NAND strings--results in (i) a reduced read-latency that approaches
that of a dynamic random access memory (DRAM) array, (ii) reduced
sensitivities to read-disturb and program-disturb conditions that
are associated with long NAND strings, and (iii) reduced power
dissipation and lower costs, relative to planar NAND or 3D NAND
arrays.
According to one embodiment of the present invention, variations in
threshold voltages within a block of NOR strings may be compensated
by providing electrically programmable reference strings within the
block. Effects on a read operation due to background leakage
currents inherent to multi-gate NOR strings can be substantially
eliminated by comparing the sensed result of the TFT being read and
that of a concurrently read TFT on a reference NOR string. In other
embodiments, the charge-storing element of each TFT may have its
structure modified to provide a high write/erase cycle endurance,
albeit a lower retention time that requires refreshing. However, as
such refreshing is required significantly less frequently than a
conventional dynamic random access memory (DRAM) circuit, the NOR
string arrays of the present invention may operate in some DRAM
applications. Such use of the NOR strings allows a substantially
lower cost-per-bit figure of merit, as compared to the conventional
DRAMs, and a substantially lower read-latency, as compared to
conventional NAND string arrays.
The present invention is better understood upon consideration of
the detailed description below, in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a shows a conceptualized memory structure 100 which
illustrates an organization of memory cells according to one
embodiment of the present invention.
FIG. 1b shows a basic circuit representation of two stacks of NOR
strings sharing common vertical word-lines, according to one
embodiment of the present invention.
FIG. 1c shows a basic circuit representation of a stack of four NOR
strings in a Y-Z plane cross section of conceptualized memory
structure 100.
FIG. 2a shows a cross section in an Y-Z plane of memory structure
200, after active layers 202-0 to 202-7 have been formed on
semiconductor substrate 201, but prior to formation of individual
active strips, in accordance with one embodiment of the present
invention.
FIG. 2b-1 shows structure 220a, which may be used to implement any
of active layers 202-0 to 202-7 of FIG. 2a, in accordance with one
embodiment of the present invention.
FIG. 2b-2 shows structure 220b, which includes additional metallic
sublayer 224 adjacent one of layers 221 and 223 of structure 220a,
in accordance with one embodiment of the present invention.
FIG. 2b-3 shows structure 220c, which includes additional metallic
sublayer 224 adjacent each of layers 221 and 223 of structure 220a,
in accordance with one embodiment of the present invention.
FIG. 2c shows a cross section in an Y-Z plane through buried
contacts 205-0 and 205-1, which connect N.sup.+ sublayer 223 in
each of active layers 202-0 and 202-1 to contacts 206-0 and 206-1
in semiconductor substrate 201.
FIG. 2d illustrates forming trenches 230 in memory structure 200 of
FIG. 2a, in a cross section in an X-Y plane through active layer
202-7 in one portion of memory structure 200 of FIG. 2a.
FIG. 2e illustrates depositing charge-trapping layers 231L and 231R
on opposite side walls of the active strips along trenches 230, in
a cross section in an X-Y plane through active layer 202-7 in one
portion of memory structure 200 of FIG. 2a.
FIG. 2f illustrates depositing polysilicon or metal 208 to fill
trenches 230.
FIG. 2g shows, after photo-lithographical patterning and etching
steps on the memory structure of FIG. 2f, local word-lines 208w and
pre-charge word-lines 208-chg are achieved by removing exposed
portions of the deposited polysilicon 208, and filling the
resulting shafts with insulation material 209 or air gap
isolation.
FIG. 2h shows a cross section in the X-Z plane through a row of
local word-lines 208w of FIG. 2g, showing active strips in active
layers 202-7 and 202-6.
FIG. 2i shows that each of local word-lines 208w of FIG. 2h is
connected to either one of global word-lines 208g-a, routed in one
or more layers provided above active layers 202-0 to 202-7, or one
of global word-lines 208g-s, routed in one or more layers provided
below the active layers between active layer 202-0 and substrate
201 (see FIG. 4a).
FIG. 2j shows an alternative embodiment to the embodiment of FIG.
2i, in which only top global word-lines--i.e., without any bottom
global word-lines--are provided, in accordance with one embodiment
of the present invention; in this embodiment, the local word-lines
along one edge of an active strip are staggered with respect to the
local word-lines on the other edge of the active strip (see FIG.
4b)
FIG. 2k shows each of local word-lines 208w controlling TFTs formed
out of the active strips on opposite sides of the local word-line,
according to one embodiment of the present invention (see FIG.
4c).
FIG. 3 illustrates the methods and circuit elements used for
setting a source voltage (V.sub.ss) on source lines in N+ sublayer
221; specifically, the source line voltage may be set through
hard-wire decoded source line connection 280 or using pre-charge
TFTs 303 and bit-line connections 270.
FIG. 4a shows a cross section in the X-Y plane, showing contacts
291 connecting local word-lines 208w to global word-lines 208g-a,
for the embodiment of the present invention shown in FIG. 2i.
FIG. 4b shows a cross section in the X-Y plane, showing contacts
291 connecting local word-lines 208w to top global word-lines
208g-a (or bottom global word-lines 208g-s) in a staggered
configuration, for the embodiment of the present invention shown in
FIG. 2j.
FIG. 4c shows a cross section in the X-Y plane, showing contacts
291 connecting local word-lines 208w to global word-lines 208g-a
and the isolation 209 between adjacent active strip-pairs, for the
embodiment of the present invention shown in FIG. 2k.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1a shows a conceptualized memory structure 100 that
facilitates illustration in this detailed description of an
organization of memory cells according to one embodiment of the
present invention. As shown in FIG. 1a, memory structure 100
represents a 3-dimensional block of memory cells formed in
deposited thin-films over the surface of substrate layer 101.
Substrate layer 101 may be, for example, a conventional silicon
wafer used for fabricating integrated circuits, familiar to those
of ordinary skill in the art. In this detailed description, a
Cartesian coordinate system (such as indicated in FIG. 1a) is
adopted solely for the purpose of facilitating discussion. Under
this coordinate system, the surface of substrate layer 101 is
considered a plane which is parallel to the X-Y plane. Thus, as
used in this description, the term "horizontal" refers to any
direction parallel to the X-Y plane, while "vertical" refers to the
Z-direction.
In FIG. 1a, each vertical column represents storage elements (i.e.,
thin-film storage transistors or TFTs) that share a vertical common
control gate or word-line in a stack of horizontal NOR strings,
with each NOR string running along the Y direction. Each NOR string
is formed out of TFTs along an "active strip", described in further
detail below. Unlike a NAND string, in a NOR string, writing,
reading or erasing one of its TFTs does not involve activation of
other TFTs in the NOR string. As shown in FIG. 1a, memory structure
100 represents an array consisting of 4 stacks of NOR strings, with
each stack having four NOR strings, and each NOR string having four
TFTs. Note that, as a conceptualized structure, memory structure
100 is merely an abstraction of certain salient characteristics of
a memory structure of the present invention. Although shown in FIG.
1a as an array of 4.times.4 NOR strings, each having four TFTs, a
memory structure of the present invention may have any number of
TFTs along any of the X, Y and Z directions. For example, there may
be 2, 4, 8, 16, 32, 64, . . . NOR-type strings along each of the X
and Z directions, with each NOR string may have 2, 4, 8, 16, . . .
8192 or more TFTs. The use of numbers that are integer powers of 2
(i.e., 2.sup.n, where n is an integer) follows a customary practice
in conventional memory design. It is customary to access each
addressable unit of memory by decoding a binary address. Thus, for
example, a memory structure within the scope of the present
invention may have M NOR strings along each of the X and Z
directions, with M being a number that is not necessarily 2.sup.n,
for any integer n. If memory block 100 has 8192 stacks of 8 NOR
strings each, with each NOR string having 8192 storage elements,
memory block 100 would have more than half a billion storage
elements in the form of NOR-type nonvolatile TFTs. As it is not
uncommon today to store more than one bit in a storage element
using a multi-level cell (MLC) technique, memory block 100 can
store more than one billion bits of information. A one-terabit
memory chip would have one thousand or more such blocks plus spare
blocks available to substitute defective or worn-out blocks.
As a conceptualized structure, memory structure 100 is not drawn to
scale in any of the X, Y, Z directions.
FIG. 1c shows a basic circuit representation of a stack of 4 NOR
strings in a Y-Z plane cross section of conceptualized memory
structure 100. As shown in FIG. 1c, each NOR string runs along the
Y direction, with storage elements connected between source line
153-m and bit lines 154-m, where m is the index between 1 to 4 of
the corresponding active strip. Corresponding storage elements in
the 4 NOR strings are connected corresponding vertical word-lines
151-n, where n is the index of the word-lines along the active
strips.
FIG. 1b shows a basic circuit representation of two stacks of NOR
strings sharing common vertical word-lines, according to one
embodiment of the present invention. The detailed structure of this
configuration is discussed and illustrated below in conjunction
with FIG. 2k. As shown in FIG. 1b, this basic circuit configuration
includes NOR strings (e.g., NOR strings 150L and 150R) that are
provided in adjacent columns of memory structure 100 sharing a
common word-line.
As shown in FIG. 1b, NOR strings 150L and 150R are NOR strings in
two active strips located on opposite sides of common word-line
151a. Storage transistors 152R-1 to 152R-4 and 152L-1 to 152L-4 are
storage elements in the four active strips to the right and in the
four active strips to the left of their common vertical word-line
151a, respectively. In this embodiment, as illustrated in greater
detail below in conjunction with FIG. 2k and FIG. 4c, a greater
storage density may be achieved by having a common local word-line
to control TFTs of adjacent active strips. For example, word-line
151n controls TFTs in the NOR strings of bit lines 153R-1, 153R-2,
153R-3 and 153R-4, and TFTs in the NOR strings of bit lines 153L-1,
153L-2, 153L-3 and 153L-4. As discussed in greater detail below, in
one embodiment, the parasitic capacitance C intrinsic to each NOR
string (e.g., the parasitic capacitance between the N+ diffusions
of a string and its numerous associated local word-lines) may be
used, under some operating conditions, to provide a virtual voltage
source.
The TFTs in the NOR strings of the present invention may be
programmed, program-inhibited, erased, or read using conventional
programming, inhibition, erasure and read voltages. In one or more
embodiments of the present invention, the TFTs are implemented by
thin-film storage transistors that are programmed or erased using
Fowler-Nordheim tunneling or direct tunneling mechanisms. In
another embodiment channel hot electron injection may be used for
programming.
FIG. 2a shows a cross section in an Y-Z plane of memory structure
200, after active layers 202-0 to 202-7 are formed on semiconductor
substrate 201, in accordance with one embodiment of the present
invention. As shown in FIG. 2a, memory structure 200 includes
active layers 202-0 to 202-7. Semiconductor substrate 201
represents, for example, a P-doped bulk silicon wafer on which
support circuits for memory structure 200 may be formed prior to
forming the active layers. Such support circuits may include both
analog and digital circuits. Some examples of such support circuits
may include shift registers, latches, sense amplifiers, reference
cells, power supply lines, bias and reference voltage generators,
inverters, Nand, Nor, Exclusive-Or and other logic gates,
input/output drivers, address decoders, including bit-line and
word-line decoders, other memory elements, sequencers and state
machines. These support circuits may be formed out of the building
blocks of conventional devices, e.g., N-Wells, P-Wells, triple
wells, N.sup.+, P.sup.+ diffusions, isolation regions, low and high
voltage transistors, capacitors, resistors, and interconnects, as
known to those skilled in the art.
After the support circuits have been formed in and on semiconductor
substrate 201, insulating layer 203-0 is provided, which may be a
deposited or grown thick silicon oxide, for example.
Next, in some embodiments, one or more layers of interconnect may
be formed, including "global word-lines," which are discussed
below. Such metallic interconnect lines (e.g., global word-line
landing pads 264 of FIG. 2c, discussed below) may be provided as
horizontal long narrow strips running along a predetermined
direction that is perpendicular to the active NOR strings to be
formed at a later step. To facilitate discussion in this detailed
description, the global word-lines are presumed to run along the X
direction. The metallic interconnect lines may be formed by
applying photo-lithographical patterning and etching steps on one
or more deposited metal layers. (Alternatively these metallic
interconnect lines can be formed using a conventional damascene
process, such as a copper damascene process). Thick oxide 203-0 is
then deposited, followed by a planarization step using conventional
chemical mechanical polishing (CMP).
Active layers 202-0 to 202-7 are then successively formed, each
active layer being insulated from previous active layer underneath
by a corresponding one of insulating layers 203-1 to 203-7. In FIG.
2a, although eight active layers are shown, any number of active
layers may be provided. In practice, the number of active layers to
provide may depend on the process technology, such as availability
of a well-controlled anisotropic etching process that allows
cutting through the active layers to reach semiconductor substrate
201. Each active layer is etched at an etching step discussed below
to form a large number of parallel active strips each running along
the Y direction.
FIG. 2b-1 shows structure 220a, which may be used to implement any
of active layers 202-0 to 202-7 of FIG. 2a, in accordance with one
embodiment of the present invention. As shown in FIG. 2b-1, active
layer 220a includes deposited polysilicon sublayers 221-223.
Sublayers 221-223 may be deposited successively in the same process
chamber without removal in between. Sublayer 223 may be formed by
depositing 5-50 nm of in-situ doped N.sup.+ polysilicon. Sublayers
222 and 221 may then be formed by depositing undoped or lightly
doped polysilicon, in the thickness range of 40-100 nm. Sublayer
221 (i.e., the top portion of the deposited polysilicon) is then
N.sup.+ doped. This N.sup.+ doping may be achieved by either (i) a
low-energy shallow ion implantation of arsenic or antimony, forming
a 20-50 nm N+ doped top sublayer 221, or (ii) in-situ doping of the
deposited polysilicon, forming a 20-50 nm N.sup.+ top sublayer 221.
(Thermal diffusion should not be used, as it would expose the lower
active layers formed earlier to greater diffusion than the upper
active layers). A low-dose implantation of boron (P-) or phosphorus
(N-) ions may also be carried out at an energy sufficient to
penetrate the implanted or in-situ N.sup.+ doped sublayer 221, so
as to adjust to an enhancement mode threshold voltage for sublayer
222 lying between top N+ doped sublayer 221 and bottom N+ doped
sublayer 223.
Thermal activation of the N+ and P- implanted species in sublayers
221 and 222 should preferably take place after all active layers
202-0 to 202-7 have been formed, using a conventional rapid thermal
annealing technique (e.g., at 700.degree. C. or higher), thereby
ensuring that all active layers experience high temperature
processing in roughly the same amount. Caution must be exercised to
limit the total thermal budget, so as to avoid merging N.sup.+
sublayer 223 with the N+ sublayer 221, so as not to eliminate
P.sup.- sublayer 222. P.sup.- sublayer 222 is required to remain
sufficiently thick to avoid N+P-N+ transistor punch-through at a
low voltage applied across N+ sublayers 221 and 223.
The final thickness of sublayer 222 represents the TFT channel
length, which may be as little as 10 nm or less over long active
strips. In one embodiment, it is possible to control the TFT
channel length to less than 10 nm by depositing an ultra-thin
(around 1 nm) film of silicon nitride (e.g., SiN or
Si.sub.3N.sub.4), or another suitable diffusion-blocking film
following the formation of N.sup.+ sublayer 223, and then again
following deposition of the polysilicon of sublayer 222 in a
thickness range between 5-30 nm, before depositing N+ polysilicon
sublayer 221. The ultra-thin silicon nitride layers can be
deposited by chemical vapor deposition, atomic layer deposition or
any other means, e.g., high pressure nitridization at low
temperature. Each ultra-thin silicon nitride layer acts as a
diffusion-barrier that prevents the N.sup.+ dopants in N.sup.+
sublayers 221 and 223 from diffusing into P.sup.- sublayer 222, yet
are sufficiently thin to only marginally impede MOS transistor
action in the region between N+ sublayer 221 (acting as a source)
and N+ sublayer 223 (acting as a drain). (Electrons in the surface
inversion layer of sublayer 222 readily tunnel directly through 1
nm of silicon nitride). These additional ultra-thin silicon nitride
layers increase the manufacturing cost, but serve to significantly
reduce leakage current in the numerous TFTs along the active strips
that are in the "off" state, while providing a high read current
for the accessed TFT that is in the "on" state.
Optionally, to provide lower resistivity along the bit lines and
the source lines of N.sup.+ sublayers 223 and 221, additional
conductive sublayer 224 may be provided adjacent the corresponding
one of N.sup.+ sublayers 221 and 223 (e.g., w in FIG. 2b-2), or
both (e.g., FIG. 2b-3). Sublayer 224 may be provided by one or more
deposited metal layers. For example, sublayer 224 may be provided
by first depositing 1-2 nm thick layer of TiN, followed by
depositing a 10-40 nm thick layer of tungsten or a similar
refractory metal, or its silicide or salicide. Reduced line
resistance is desirable for reducing the "RC delay" of a signal
traversing a long conductive strip (i.e., the time delay due to the
product of the line resistance R and the line capacitance C), and
for minimizing the "IR drop" across a long and narrow active strip
(i.e., the voltage drop due to the product of the current I and the
line resistance R). Inclusion of metal sublayer 224 in each of
active layers 202-0 to 202-7 may, however, increase cost and
complexity in the manufacturing process, including the complication
that some of the metallic materials are relatively more difficult
to anisotropically etch than materials such as polysilicon or
silicon oxide in the other sublayers. However, the use of metal
sublayer 224 enables use of considerably longer active strips which
result in superior array efficiency. On the other hand, shorter
active strips have superior immunity to leakage between N.sup.+
sublayer 223 and N.sup.+ sublayer 221, and lower intrinsic
capacitance than the longer strips. The integrated circuit designer
may opt for a shorter active strip (with or without one or both
metal layers) when low latency is most valued. Alternatively the
strip resistance may be reduced by providing buried contacts at
both ends, of each active strip, rather than just at one end.
Block-formation patterning and etching steps define separate blocks
in the active layers formed. Each block defines an area in which a
large number (e.g., thousands) of active strips running in parallel
along the Y direction may be formed, as discussed below, with each
active strip eventually forming a large number (e.g., thousands) of
TFTs.
Each of active layers 202-0 to 202-7 is successively formed, with
each active layer being formed by repeating the steps described
above. In addition, in the block-formation patterning that defines
the blocks of each active layer, each next higher active layer
extends slightly beyond the previous active layer (see, e.g., as
illustrated in FIG. 2c, discussed below, layer 202-1 extends beyond
layer 202-0) to allow the higher active layer to access its
specific decoders and other circuitry in semiconductor substrate
201 through designated buried contacts.
FIG. 2c shows a cross section in an Y-Z plane through buried
contacts 205-0 and 205-1, which connect N.sup.+ sublayer 223 in
each of active layers 202-0 and 202-1 to contacts 206-0 and 206-1
in semiconductor substrate 201. As shown in FIG. 2c, buried
contacts 205-0 and 205-1 connect contacts 206-0 and 206-1 in
semiconductor substrate 201, for example, to the local bit or
source lines formed out of N.sup.+ sublayer 223 in each of active
layers 202-0 and 202-1. Buried contacts for active layers 202-2 to
202-7 (not shown) may be similarly provided to connect active
layers 202-2 to 202-7 to contacts 206-2 to 206-7 (not shown) in
semiconductor substrate 201. Through a switch circuit, each of
contacts 206-0 to 206-7 may apply a pre-charge voltage V.sub.b1 to
the respective bit line or source line or, during a read operation,
may be connected to an input terminal of a sense amplifier or a
latch. The switch circuit may selectively connect each of contacts
206-0 to 206-7 to any of a number of specific voltage sources, such
as a programming voltage (V.sub.program), an inhibit voltage
(V.sub.inhibit), an erasure voltage (V.sub.erase), or any other
suitable predetermined or pre-charge reference voltage V.sub.b1 or
V.sub.ss. In one embodiment, discussed below, using the relatively
large parasitic capacitance C along a bit line or source line, a
virtual ground may be created in sublayer 221 of each active layer.
In that embodiment, buried contacts and separate interconnects need
not be provided to the bit or source lines formed out of N.sup.+
sublayer 221 in each of active layers 202-0 to 202-7.
FIG. 2c also shows buried contacts 261-0 to 261-n for connecting
global word-lines --which are to be formed running along the X
direction--to contacts 262-0 to 262-n in semiconductor substrate
201. These global word-lines are provided to connect corresponding
local word-lines 208w to be formed (see, e.g., FIG. 2g, which is
described below). Landing pads 264 are provided to allow connection
to local word-lines 208w, which are yet to be formed vertically on
top of global word-lines 261-0 and 261-n. Through a switch circuit
and a global word-line decoder, each of global word-lines 262-0 to
262-n may be selectively connected, either individually, or shared
among several global word-lines to any one of a number of reference
voltage sources, such as stepped programming voltage
(V.sub.program), a read voltage (V.sub.read) and an erasure voltage
(V.sub.erase).
These buried contacts, the global word-lines and the landing pads
may be formed using conventional photo-lithographical patterning
and etching steps, following by deposition of one or more
conductors or by alloying (e.g., tungsten metal or tungsten
silicide).
After the top active layer (e.g., active layer 202-7) is formed,
trenches are created by etching through the active layers to reach
the bottom global word-lines (or semiconductor substrate 201) using
a strip-formation mask. The strip-formation mask consists of a
pattern in a photoresist layer of long narrow strips running along
the Y direction (i.e., perpendicular to that of global word-line
strips that run along the X direction). Sequential anisotropic
etches etch through active layers 202-7 down to 202-0, and
dielectric isolations layers 203-7 down to 203-0. As the number of
active layers to be etched, which is eight in the example of FIG.
2c (and, more generally may be 16 or more active layers), a
photoresist mask by itself may not be sufficiently robust to hold
the strip pattern through the numerous etches necessary to etch
through the lowest active layer. Thus, reinforcement by a hard mask
material, such as carbon, may be required, as is known to those of
ordinary skill in the art. Etching terminates at the dielectric
isolation above the landing pads of the global word-lines. It may
be advantageous to provide an etch-stop barrier film such as
aluminum oxide to protect the landing pads during the trench etch
sequence.
FIG. 2d illustrates forming trenches 230 in memory structure 200 of
FIG. 2a, in a cross section in an X-Y plane through active layer
202-7 in one portion of memory structure 200 of FIG. 2a. Between
adjacent trenches 230 is a stack of high aspect-ratio, long and
narrow active strips. To achieve the best etch result, etch
chemistry may have to be changed when etching through the materials
of the different sublayers, especially when metal sublayers 224 are
present. The anisotropy of the multi-step etch is important, as
undercutting of any sublayer should be avoided as much as possible,
and so that an active strip in the bottom active layer (e.g., an
active strip in active layer 202-0) has approximately the same
width and gap spacing to an adjacent active strip as the
corresponding width and gap spacing in an active strip of the top
active layer (i.e., an active strip of active layer 202-7).
Naturally, the greater the number of active layers in the stack to
be etched, the more challenging is the design of the successive
etches. To alleviate the difficulty associated with etching
through, for example, 32 active layers, etching may be conducted in
sections of, say 8 layers each, as discussed in Kim, mentioned
above, at pp. 188-189. As shown in FIG. 2d, trenches 230 run along
the Y direction.
Thereafter, one or more layers of charge-trapping material are
conformally deposited on the sidewalls of the active strips in
trenches 230. The charge-trapping layer is formed by first
depositing or growing a thin tunneling dielectric film of a 2-10 nm
thickness, typically a silicon dioxide layer or a silicon
oxide-silicon nitride-silicon oxide ("ONO") triple layer, followed
by deposition of a 4-10 nm thick layer of charge-trapping material,
typically silicon nitride or silicon-rich nitride or oxide or
nanocrystals or nanodots embedded in a thin dielectric film, which
is then capped by a blocking dielectric. The blocking dielectric
may be a 5-15 nm thick layer consisting, for example, of an ONO
layer, or a high dielectric constant film, such as aluminum oxide,
hafnium oxide or some combination thereof. The storage element can
be SONOS, TANOS, nanodot storage, isolated floating gates or any
suitable charge-trapping sandwich structures known to those of
ordinary skill in the art. Trenches 230 must be sufficiently wide
to accommodate the storage elements on the two opposing sidewalls
of the adjoining active strips, plus the vertical local word-lines
to be shared by the TFT's on these opposite sidewalls. FIG. 2e
illustrates deposited charge-trapping layers 231L and 231R on
opposite side walls of the active strips along trenches 230, in a
cross section in an X-Y plane through active layer 202-7 in one
portion of memory structure 200 of FIG. 2a.
Contact openings at the bottom global word-lines are
photo-lithographically patterned at the top of layer 202-7 and
exposed by anisotropically etching through the charge-trapping
materials at the bottom of trenches 230, stopping at the bottom
global word-line landing pads (e.g., global word-line landing pads
264 of FIG. 2c). In one embodiment, to be described in conjunction
with FIG. 2i below, only alternate rows of trenches 230 (e.g., the
rows in which the word-lines formed therein are assigned
odd-numbered addresses) should be etched down to the bottom global
word-lines. In some embodiments, etching is preceded by a
deposition of an ultra-thin film of polysilicon (e.g. 2-5 nm thick)
to protect the vertical surface of the blocking dielectric on the
sidewalls of trenches 230 during the anisotropic etch of the
charge-trapping material at the bottom of trenches 230.
Thereafter, doped polysilicon (e.g., P.sup.+ polysilicon) may be
deposited over the charge-trapping layers to form the control gates
or vertical local word-lines. P.sup.+ doped polysilicon is
preferable because of its higher work function than N+ doped
polysilicon. Alternatively, a metal with a high work function
relative to SiO.sub.2 (e.g., tungsten, tantalum, chrome or nickel)
may also be used to form the vertical local word lines. Trenches
230 may now be filled with the P.sup.+ doped polysilicon or the
metal. In the embodiment of FIG. 2i, discussed below, the doped
polysilicon or metal in alternate rows of trenches 230 (i.e., the
rows to host local word-lines that are assigned odd-numbered
addresses) is in ohmic contact with the bottom global word-lines.
The polysilicon in the other ones of trenches 230 (i.e., the rows
to host local word-lines that are assigned even-numbered addresses)
are isolated from the bottom global word-lines. (These local
word-lines are to be contacted by top global word-lines routed
above the top active layer). The photoresist and hard mask may now
be removed. A CMP step may then be used to remove the doped
polysilicon from the top surface of each block. FIG. 2f illustrates
depositing polysilicon 208 to fill trenches 230.
FIG. 2g shows, after photo-lithographical patterning and etching
steps on the memory structure of FIG. 2f, local word-lines 208w are
achieved by removing exposed portions of the deposited polysilicon
208, and filling the resulting shafts with insulation material 209.
As the removal of the doped polysilicon in this instance is a high
aspect-ratio etch in a rather confined space, a hard mask may be
required, using the technique described above. The resulting shafts
may be filled with insulation material 209 or left as an air gap.
The mask pattern that exposes the doped polysilicon for excavation
are parallel strips that run along the X direction, so that they
coincide with the global word-lines that are required to be formed
in one embodiment to contact local word-lines 208w.
In FIG. 2g, the portions of charge-trapping layers 231L and 231R
adjacent insulation material 209 remained after the removal of the
corresponding portions of deposited polysilicon 208. In some
embodiments, those portions of charge-trapping layers 231L and 231R
may be removed by a conventional etching process prior to filling
the shafts with insulation material 209. Etching of the
charge-trapping materials in the shafts may be carried out
concurrently with the removal of the doped polysilicon, or
subsequent to it. A subsequent etch would also remove any fine
polysilicon stringers that the anisotropic etch has left behind;
such polysilicon stringers may cause undesirable charge leakage,
serving as resistive leakage paths between adjacent vertical local
word-lines. Removal of such charge trapping material also
eliminates lateral diffusion of trapped charge between one TFT and
the TFTs immediately to its left and right along the same
string.
FIG. 2h shows a cross section in the X-Z plane through a row of
local vertical word-lines 208w (shown also in FIG. 2g in the X-Y
plane), showing active strips in active layers 202-7 and 202-6. As
shown in FIG. 2h, each active layer includes N.sup.+ sublayer 221,
P.sup.- sublayer 222, and N.sup.+ sublayer 223. In one embodiment,
N+ sublayer 221 (e.g., a source line) is connected to a ground
reference voltage V.sub.ss (not shown) and N.sup.+ sublayer 223
(e.g., a bit line) is connected to a contact in substrate 201
according to the method illustrated in FIG. 2c. Thus, local
word-line 208w, the portion of active layer 202-7 or 202-6 facing
word-line 208w and the charge-trapping layer 231L between word-line
208w and that portion of active layer 202-7 or 202-6 form a storage
element or storage TFT, as indicated by reference numeral 281 and
282 in FIG. 2h. Facing TFTs 281 and 282 on the opposite side of
208W are TFTs 283 and 284 respectively, incorporating therein
charge trapping layer 231R. On the other side of the active strips
202-6 and 202-7 providing TFTs 283 and 284 are TFTs 285 and 286.
Accordingly, the configuration shown in FIG. 2h represents the
highest packing density configuration for TFTs, with each vertical
word-line shared by the two active strips along its sides, with
each active strip being shared by the two word-lines along its
sides. N.sup.+ sublayer 223 can be charged to a suitable voltage
required for an operation of the storage transistors at hand (e.g.,
program voltage V.sub.prog, inhibition voltage V.sub.inhibit, erase
voltage V.sub.erase, or the read reference voltage V.sub.b1). As
shown in FIG. 2h, additional metallic sublayer 224 increases the
conductivity of the bit line, so as to facilitate memory device
operations. In another embodiment, N.sup.+ sublayer 221 in any of
active layers 202-0 to 202-7 may be left floating. In each active
layer, one or more of the local vertical word-lines (referred to as
a "pre-charge word-line"; e.g., pre-charge word-lines 208-chg in
FIG. 2g) may be used as a non- memory TFT. When a suitable voltage
is applied (i.e., rendering the pre-charge TFT to the "on" state),
each pre-charge word-line momentarily inverts sublayer 222, so that
N.sup.+ sublayer 221 may be pre-charged to the voltage V.sub.ss on
N.sup.+ sublayer 223. When the voltage on the pre-charge word-line
is withdrawn, (i.e., returned to its "off " condition) and all the
other word-lines on both sides of the strip are also "off", device
operation may proceed with N.sup.+ sublayer 221 left electrically
charged as a virtual reference at the pre-charged voltage V.sub.ss
because the parasitic capacitance in the strip capacitor of N.sup.+
sublayer 221 is large enough to hold its charge sufficiently long
to support program and read operations (see below).
Each local word-line 208w may be used to read, write or erase the
charge stored in the designated one of the TFTs formed in each of
active layers 202-0 to 202-7, located on either charge-trapping
portion 231L or 231R, when a suitable voltage is imposed.
Alternatively, in one embodiment, to be described in conjunction
with FIG. 2k below, each local word-line 208w may be used to read,
write or erase the charge stored in any of the TFTs formed in each
of active layers 202-0 to 202-7, located on either charge-trapping
portions 231L or 231R, when a suitable voltage is imposed. However,
as shown in FIG. 2k, only one of the two sides of active layers
202-0 to 202-7 is formed as storage TFTs, thereby eliminating the
need for both bottom and top global word-lines in the
embodiment.
An isolation dielectric or oxide may be then deposited and its
surface planarized. Contacts to semiconductor substrate 201 and to
local word-lines 208w may then be photo-lithographically patterned
and etched. In one embodiment, which is described in conjunction
with FIG. 2i and corresponding FIG. 4a, contacts to local
word-lines 208w are provided only for those assigned an
even-numbered address (local word-lines assigned an odd address are
contacted from the bottom of the array by bottom global
word-lines). For the embodiment shown in FIG. 2j, contacts are
provided for every local word-line but the local word-lines are
staggered relative to opposing word-lines, as shown in FIG. 4b. A
deposited metal layer provides a top metal layer and the contacts.
Such a metal layer may be provided by, first, forming a thin TiN
layer, followed by forming a low resistance metal layer (e.g.,
tungsten). The metal layer is then photo-lithographically patterned
to form top global word-lines. (Alternatively, these global
word-lines may be provided by a copper damascene process.) In one
embodiment, these global word-lines are horizontal, running along
the X direction, electrically connecting with the contacts formed
in the isolation oxide (i.e., thereby contacting local word-lines
208w) and with the contacts to semiconductor substrate 201 (not
shown). Of course other mask and etch process flows known to those
skilled in the art are possible to form even and odd addressed
local word-lines and connect them appropriately to their global
word-lines, either from the top of the array through the top global
word-lines or from the bottom of the array through the bottom
global word-lines and in some embodiments from both.
FIG. 2i shows that each of local word-lines 208w of FIG. 2h is
connected to either one of global word-lines 208g-a, routed in one
or more layers provided above active layers 202-0 to 202-7, or one
of global word-lines 208g-s, routed in one or more layers provided
below the active layers between active layer 202-0 and substrate
201. Local word-lines 208w that are coupled to bottom global
word-lines may be assigned odd addresses, while local word-lines
208w coupled to the top global word-lines may be assigned even
addresses. FIG. 4a shows a corresponding cross section in the X-Y
plane, showing contacts 291 connecting local word-lines 208w to
global word-lines 208g-a. (In contrast, in the embodiment of FIG.
2k and corresponding FIG. 4c, local word-lines 208w control each
active strip on only one of the active strip's sides.)
FIG. 2j shows an alternative embodiment to the embodiment of FIG.
2i, in which only top global word-lines are provided (or,
alternatively, only bottom global word-lines are provided), in
accordance with one embodiment of the present invention. In this
embodiment, the local word-lines along one edge of an active strip
are staggered with respect to the local word-lines on the other
edge of the active strip. This is shown in FIG. 4b, which shows a
corresponding cross section in the X-Y plane, showing contacts 291
connecting local word-lines 208w to top global word-lines 208g-a
(or bottom global word-lines 208g-s) in a staggered configuration.
This embodiment simplifies the process flow by obviating the
process steps needed to form the bottom global word-lines (or the
top global word-lines, as the case may be). In the embodiment of
FIG. 2i and corresponding FIG. 4a, in which both top and bottom
global word-lines are provided, two TFTs may be provided in each
active layer of each active strip within one pitch of a global
word-line (i.e., in each active strip, one TFT is formed using one
sidewall of the active strip, and controlled from a bottom global
word-line, the other TFT is formed using the other sidewall of the
active strip, and controlled from a top global word-line). (A pitch
is one minimum line width plus a required minimum spacing between
adjacent lines). In contrast, as shown in FIG. 2j and corresponding
FIG. 4b, only one TFT may be provided within one global word line
pitch in each active layer. The two local word-lines 208w at the
two sides of each strip can be staggered relative to each other so
that two global word-line pitches are required to contact them
both. The penalty for the staggered embodiment is the forfeiting of
the double-density TFT inherent in having both edges of each active
strip providing a TFT within one pitch of each global
word-line.
FIG. 2k shows each of local word-lines 208w controlling TFTs formed
out of the active strips on opposite sides of the local word-line,
according to one embodiment of the present invention. FIG. 4c shows
a corresponding cross section in the X-Y plane, showing contacts
291 connecting local word-lines 208w to global word-lines 208g-a
and the isolation 209 between adjacent active strip-pairs. As shown
in FIG. 2k, each TFT is formed from either one of a dual-pair of
active strips located on opposite side of a common local word-line,
with each dual-pair of active strips being isolated by a trench
filled with oxide or dielectric material or an air gap 209 from
similarly formed adjacent dual-pairs of active strips. The
isolation trenches between adjacent dual-pairs of active strips
accommodate the charge-trapping-material 231 or polysilicon 208.
After the local word-lines 208w are defined by etching, the
protected isolation trenches may then be filled with oxide or
dielectric material 209, or left as air gaps.
FIG. 3 shows schematically an embodiment where sublayer 221 of each
active strip (see, e.g., FIG. 2b-1) is connected by hard wires 280
(dashed line) to a source reference voltage V.sub.ss by a metal or
N+ doped polysilicon conductor. Each of hardwires 280 may be
independently connected, so that the source voltages for different
layers need not be the same. Since sublayer 221 is formed only
after sublayer 223 is formed, the metal or N+ doped polysilicon
conductor to connect sublayer 221 to the reference voltage V.sub.ss
requires one or two additional patterning and etching steps to each
of active layers 202-0 to 202-7, hence increasing the processing
cost. To avoid this added cost, use is made of the large intrinsic
parasitic capacitance C of each active NOR string. Taking advantage
of intrinsic parasitic capacitance C, hard wires 280 are not needed
and sublayer 221 of each active strip is left floating, after being
temporarily pre-charged to voltage V.sub.ss transferred to it from
bit-line sublayer 223 through the action of local vertical
pre-charge TFTs that are controlled by pre-charge word-lines
208-chg. In a long horizontal NOR string (e.g., with 1,024 or more
memory TFTs), several pre-charge TFTs may be provided on either
side of the active strip (e.g., one every 512 TFTs). Assuming a
local capacitor between each local word-line as one plate and
N+/P-/N+ active layer as the other plate, each such TFT provides a
capacitor that is typically around 3.times.10.sup.18 farads. As
there are slightly more than 2,000 TFTs contributing capacitance
from both sides of the strip, the total capacitance C of the string
is approaching 0.01 picofarad, which is sufficient to preserve the
pre-charge voltage on it well beyond the milliseconds required for
write, erase or read operations that follow immediately after a
pre-charging operation. Capacitance C may be increased by
lengthening the NOR string to accommodate thousands more TFTs along
each side of the string, correspondingly increasing the retention
time of pre-charge voltage V.sub.ss on N.sup.+ sublayer 221.
However, a longer NOR string suffers from an increased leakage
current between N.sup.+ sublayer 221 and N.sup.+ sublayer 223, such
leakage current may interfere with the sensed current when reading
the one TFT being addressed. Also, the potentially longer time it
takes to pre-charge a larger capacitor during a read operation can
conflict with the requirement for a low read latency (i.e., a fast
read access time). To speed up the pre-charging of the capacitance
C of a long NOR string would typically require providing more than
one pre-charge TFT; such pre-charge TFTs may be distributed
throughout the length of the NOR string.
Also shown in FIG. 3 is an optional connection 290 to the P-
sublayers 222 to access a back-bias voltage V.sub.bb from substrate
201. A negative V.sub.bb voltage can be used to modulate the
threshold voltage of the TFTs along each active strip, so as to
reduce the subthreshold leakage current between N+ source sublayer
221 and N+ drain sublayer 223. In some embodiments a high positive
V.sub.bb voltage can be applied during an erase operation to
tunnel-erase TFTs whose control gates are held at ground
potential.
Because the TFTs in a NOR string are connected in parallel, the
read operating condition for the NOR strings of the present
invention should preferably ensure that all TFTs along both edges
of an active strip operate in enhancement mode (i.e., they each
have a positive threshold voltage between their control gate 151n
and their source 221 voltage V.sub.ss), so as to suppress the
leakage current between N.sup.+ sublayers 221 and 223 of the active
strip when all control gates on both sides of the strip are held
at, or below V.sub.ss. This enhancement threshold voltage can be
achieved by doping sublayer 222 with a P- dopant concentration
(typically by boron, in a concentration typically between
1.times.10.sup.16 and 1.times.10.sup.17 per cm.sup.3), so as to
result in a native TFT threshold voltage of around 1 volt, and by
holding all unaddressed local word-lines on both sides of the
active strip at 0 volt. Alternatively, if some of the TFTs along an
active string have negative threshold voltage (i.e in depletion
mode threshold voltage), leakage current suppression can be
achieved by raising the V.sub.ss voltage on N.sup.+ sublayer 221 to
around 1.5 volts and the voltage V.sub.b1 on N.sup.+ sublayer 223
to a voltage that is about half a volt to one volt above 1.5 volts,
while holding all local word-lines at 0 volt. This provides the
same effect as holding the word-line voltage at -1.5 volts with
respect to the source, thereby suppressing any leakage due to TFTs
that are in a slightly depleted threshold voltage. Also, after
erasing a NOR string, the erase operation should preferably include
a soft-programming operation that shifts any TFT that has been
over-erased into a depletion mode threshold voltage back into an
enhancement mode threshold voltage.
The charge-trapping material (e.g., an ONO stack) described above
has a long data retention time (typically measured in many years),
but low endurance. Endurance, which is a measure of a storage
transistor's performance degradation after some number of
write-erase cycles, is typically considered low if it is less than
ten thousand cycles. However, one may alter the charge-trapping
material to reduce retention times, but significantly increase
endurance (e.g., reducing retention times to hours, while
increasing endurance to tens of millions of write/erase cycles).
For example, in an ONO film or a similar combination of
charge-trapping layers, the tunnel dielectric, typically 6-8 nm of
silicon oxide can be reduced to 2 nm or less of silicon oxide, or
replaced altogether with another dielectric (e.g., silicon nitride
or SiN). Under a modest positive control gate voltage, electrons
will be attracted through direct tunneling (as distinct from
Fowler-Nordheim tunneling) into the silicon nitride charge-trapping
layer where the electrons will be temporarily trapped for a few
minutes to hours or days. The charge-trapping silicon nitride layer
and the blocking layer of silicon oxide or aluminum oxide will keep
these electrons from escaping to the control gate word-line, but
they will eventually leak back out to the active sublayers
(electrons are negatively charged and repel each other). Even if
the 2 nm or less tunnel dielectric breaks down locally after
extended cycling, the trapped electrons will be slow to depart from
their traps in the silicon nitride layer. Other combinations of
charge storage materials may also result in a high endurance but
low retention ("semi-volatile") TFT. Such a TFT may require
periodic write refresh to replenish the lost charge. Because such a
TFT provides a relatively fast read access time with a low latency,
NOR string arrays of the present invention that have such TFTs may
be useful in applications that currently can get by with relatively
slow DRAMs. The advantages of such NOR string arrays over DRAMs
include: a much lower cost-per-bit because DRAMs cannot be built in
three dimensional blocks, and a much lower power dissipation, as
the refresh cycles need only be run approximately once every few
minutes or once every few hour, as compared to every few
milliseconds required in current DRAM technology. The NOR string
arrays of the present invention are achieved by changing the
constitution of the charge-trapping material (e.g., charge-trapping
layers 231L and 231R in FIG. 2e), and by appropriately adapting the
program/read/erase conditions to incorporate the periodic data
refreshes.
According to another embodiment of the present invention, NOR
string arrays may also be programmed using a channel hot-electron
injection approach, similar to that which is used in NROM/Mirror
Bit transistors, known to those of ordinary skill in the art. In an
NROM/Mirror Bit transistor, charge representing one bit is stored
at one end of the channel region next to the junction with the
drain region, and by reversing polarity of the source and drain,
charge representing a second bit is programmed and stored at the
opposite end of the channel region next to the source junction.
Typical programming voltages are 5 volts on the drain, 0 volt on
the source and 8 volts on the control gate. Reading both bits
requires reading in reverse the source and drain, as is well known
to those skilled in the art. However, channel hot electron
programming is much less efficient than tunnel programming, and
therefore the method does not lend itself to the massively parallel
programming possible by tunneling. The channel hot electron
injection approach provides double the bit density though, making
it attractive for applications such as archival memory.
Exemplary operations for the NOR strings of the present invention
are next described.
Read Operation
To read a TFT among the many TFTs on an active strip, the TFTs on
both sides of the active strip are initially set in an "off" state,
so that all global and local word-lines in a selected block are
initially held at 0 volts. In FIG. 3, the addressed NOR string can
either share a sensing circuit among several NOR strings through a
decoding circuitry, or each NOR string may be directly connected to
a dedicated sensing circuit and many other addressed NOR strings
sharing the same plane can be sensed in parallel. Each addressed
NOR string has its source (N.sup.+ sublayer 221) set at
V.sub.ss.about.0V, either through hard-wire 280 or through bit-line
connection 270 in conjunction with pre-charge word-line 208-chg (in
this case V.sub.b1 is initially held at 0 volt during a pre-charge
phase). After the pre-charge phase, the bit-line (i.e., N.sup.+
sublayer 223) is then set to around V.sub.b1.about.2 volts through
bit-line connection 270. The V.sub.b1 voltage is the sense voltage
at the sense amplifiers for the addressed NOR strings. The one
addressed global word-line and all its associated vertical local
word-lines are raised from 0 volts to typically around 2 volts
while all other global word lines in the block are in their off
state. If the addressed TFT is in an erased state (i.e.,
V.sub.th.about.1 volt), bit-line voltage V.sub.b1 will begin to
discharge toward source voltage V.sub.ss. This voltage dip is
detected by the respective sense amplifier. However, if the
addressed TFT is in a programmed state (e.g, V.sub.th.about.3
volts) no dip is detected.
When MLC is used (i.e., more than one bit of information is stored
in each TFT), the addressed TFT may be programmed to any of several
threshold voltages (e.g., 1 volt (for an erased state), 2.5 volts,
4 volts or 5.5 volts for the four states representing two bits of
data). The addressed global word-line and its local word-lines can
be raised in incremental voltage steps until conduction is detected
in the addressed TFT by the respective sense amplifier.
Alternatively a single word-line voltage V.sub.b1 can be applied
(e.g. V.sub.b1=6 volts), and the rate of discharge of voltage
V.sub.b1 can be compared with the rate of discharge of each of
several programmable reference voltages representative of the four
voltage states representing the bits stored in the TFT. This
approach can be extended to store eight states (for 3-bit MLC TFT)
or a continuum of states, thereby effectively providing analog
storage. The programmable reference voltages are stored on NOR
strings dedicated as reference NOR strings in the same block,
preferably located in the same plane as the addressed NOR string.
When MLC is used, more than one programmable reference NOR string
may be provided to detect each of the programmed states. For
example, if 3-bit MLC is used, there should be at least seven
reference NOR strings; preferably, an entire set of reference NOR
strings should be provided for each active layer and each block.
The programmable reference NOR strings closely track the
characteristics of the operating NOR strings in the same block
through read, program, and background leakage. Only TFTs on one of
the two sides of the active strip can participate in a read
operation; each TFT on the other side of an active strip must be
set to the "off" state. Other ways to read the correct state of a
multi-state TFT, as are known to those skilled in the art.
Reading is fast because, in a NOR string, only the TFT to be read
is required to be "on", as compared to a NAND string, in which the
TFTs in series with the one TFT being read must also be "on". In
the embodiments in which metal sublayer 224 is not provided in the
active layer (see, e.g., 220a of FIG. 2b-1), for a string with
1,024 TFTs on each side, a typical resistance R is .about.100,000
Ohm and a typical capacitance C.about.10.sup.14 farad, to provide a
RC time delay in the order of one nanosecond. Even with 4,098 TFTs
in each NOR string on either side of an active strip, the RC time
delay would still be less than 20 nanoseconds. The time delay may
be much reduced, if metal sublayer 224 is provided to reduce the
resistance R of the active strip. To further reduce read latency,
some or all the planes in selected active blocks may be kept always
pre-charged to their read voltages V.sub.ss and V.sub.b1, thereby
rendering them ready to immediately sense the addressed TFT (i.e.,
obviating the pre-charge phase immediately before the read
operation). Such ready-standby requires very little standby power
because the current required to periodically re-charge capacitor C
to compensate for charge leakage is very small. Within each block,
all strings on all eight or more planes can be pre-charged to be
ready for fast read; for example, after reading all strings in
plane 207-0 (FIG. 2a), plane 207-1 can be read in short order
because its V.sub.ss and V.sub.b1 are already previously set for a
read.
In memory block 100, only one TFT per NOR string can be read in a
single operation. In a plane with eight thousand NOR strings, the
eight thousand TFTs that share a common global word-line may all be
read concurrently, provided that each NOR string is connected to
its own sense amplifier. If each sense amplifier is shared among,
for example, four NOR strings in the same plane using a string
decode circuit, then four read operations are required to take
place in four successive steps, with each read operation involving
two thousand TFTs. Each plane can be provided its own set of
dedicated sense amplifiers or, alternatively one set of sense
amplifiers can be shared among NOR strings in the eight or more
planes through a plane-decoding selector. Providing separate sense
amplifiers for each plane allows concurrent read operations of NOR
strings of all planes, which correspondingly improves the read
throughput. However, such throughput comes at the expense of the
extra chip area needed for the additional sense amplifiers, and may
also create ground voltage bounces when too many TFTs are read all
at once. In that regard, embodiments relying on pre-charged
capacitor C to set the virtual V.sub.ss voltage are particularly
advantageous, as it eliminates such ground voltage bounces because
source voltage V.sub.ss of all NOR strings is not connected to the
chip's V.sub.ss ground line.
Program (Write) and Program-Inhibit Operations
There are several ways to program an addressed TFT to its intended
threshold voltage. The most common way is by tunneling, i.e.,
either direct tunneling or Fowler-Nordheim tunneling. Either one of
these tunneling and charge-trapping mechanisms is highly efficient,
so that very little current is needed to program a TFT, allowing
parallel programming of tens of thousands of TFTs with minimal
power dissipation. For illustration purpose, let us assume that
programming by tunneling requires a 20 volts pulse of 100
microseconds (us) duration to be applied to the addressed word-line
(control gate), with 0 volts applied to the active strip (e.g.,
202-0 in FIG. 2a). Under these conditions N+ source and drain
(sublayers 221, 223 in FIG. 2b-1) and the P- channel (sublayer 222)
of the TFT is inverted at the surface and electrons tunnel into the
charge trapping layer. TFT Programming can be inhibited by applying
a half-select voltage (e.g., 10 volts, in this example).
Program-inhibit can be accomplished, for example, either by
lowering the word-line voltage to 10 volts while keeping the strip
voltage at 0 volt, or by raising to 10 volts the active strip
voltage, while keeping the word-line voltage at 20 volts, or some
combination of the two. Only one TFT on an addressed active strip
can be programmed at one time, but TFTs on other strips can be
programmed in parallel in the same programming cycle. When
programming one of the many TFTs on one side of an addressed active
strip (e.g., one TFT in the even-addressed NOR string), all other
TFTs in the NOR strings are programming-inhibited, as are all TFTs
on the other side of the active strip (e.g., all TFTs in the
odd-addressed NOR string). Once the addressed TFT is programmed to
the target threshold voltage of its designated state,
programming-inhibition of that TFT is required, as overshooting
that target voltage will exert unnecessary stress on the TFT. When
MLC is used, overshooting the target voltage may cause overstepping
or merging with the threshold voltage of the next higher target
threshold voltage state. It should be noted that all TFTs in the
adjacent active strips on the same plane sharing the same global
word-line and its associated local word-lines--thus, are exposed to
the 20 volts programming voltage--and are required to be
programming-inhibited once they have been programmed to their
target threshold voltages. Similarly, all TFTs on other planes that
are within the same block and that share the same global word-line
and its associated local word-lines--and thus, are also exposed to
the 20 volts programming voltage--are also required to be
programming-inhibited. These program and program-inhibit conditions
can all be met under the present invention because the even and odd
sides of each active strip are controlled by different global
word-lines and their associated local word-lines, and because the
voltage on each active strip regardless of its plane can be set
independently from all other active strips or other planes.
In one example, all TFTs in a block are first erased to a threshold
voltage of around 1 volt. The voltage on the active strip of each
addressed TFT is then set to 0 volts (e.g., through connection 270
in conjunction with pre-charge word-line 208-chg, or through
connection 280, as illustrated in FIG. 3), if the addressed TFT is
to be programmed; otherwise, the voltage on the active strip of the
addressed TFT is set to 10 volts if it is to remain in its erased
state (i.e., program-inhibited). The global word-line associated
with the addressed TFT is then raised to 20 volt, either in one
step or in short-duration steps of incrementally increasing
voltages, starting at around 14 volts. Such incremental voltage
steps reduce the electrical stress across the TFT and avoid
overshooting the target threshold voltage. All other global
word-lines in the block are set at half-select 10 volt. All active
strips on all planes that are not being addressed in the block, as
well as all active strips within the addressed plane that are not
individually addressed, are also set at 10 volts, or may be
floated. These active strips are strongly capacity-coupled to the
local word-lines, which are at 10 volts, and thus float at close to
10 volts. Each of the incrementally higher voltage programming
pulses is followed by a read cycle to determine if the addressed
TFT has reached its target threshold voltage. When the target
threshold voltage is reached, the active strip voltage is raised to
10 volts (alternatively the strip is floated, and rises close to 10
volts when all but the one addressed global word lines in the block
are raised to 10 volt) to inhibit further programming, while the
global word-line keeps programming other addressed strips on the
same plane that have not yet attained their target threshold
voltages. This programming sequence terminates when all addressed
TFTs have been read-verified to be correctly programmed. When MLC
is used, programming of the correct one of the multiple threshold
voltage states can be accelerated by first pre-charging capacitor C
of all addressed active strips (see, e.g., through connection 270
and pre-charge word-line 208-chg of FIG. 3) to one of several
voltages (e.g., 0, 1.5, 3.0, or 4.5 volts, when two bits of
information are to be stored in each TFT). 20 volt pulses are then
applied to the addressed global word-line, which expose the TFT to
different effective tunneling voltages (i.e., 20, 18.5, 17, or 15.5
volts, respectively), resulting in the correct one of the four
threshold voltages being programmed in a single course programming
step. Thereafter, fine programming pulses may then be applied at
the individual TFT level.
Because of the intrinsic capacitance C of every active strip in the
block, all active strips on all planes in a block can have their
pre-charge voltage states set in place in advance of applying the
high voltage pulsing on the addressed global word-line.
Consequently, concurrent programming of a great many TFTs can be
achieved. Thereafter, individual read-verify, and where necessary,
resetting properly programmed active strips into program-inhibit
mode can be carried out. Pre-charging is advantageous, as
programming time is relatively long (e.g., around 100 microsecond)
while pre-charging all capacitors C or read-verifying of addressed
TFTs can be carried out over a time period that is more than 1,000
times shorter. Thus, it is advantageous to program as many TFTs as
possible in a single global word-line programming sequence.
Erase Operation
With some charge-trapping layers, erase is accomplished through
reverse-tunneling of the trapped charge, which can be rather slow
(e.g., may require tens of milliseconds of erase pulsing).
Therefore, the erase operation is frequently implemented at the
block level, often in the background. A typical block may be eight
planes high with each plane having 8,000 active strips each with
4,000 TFTs on either of its sides, for a total of half a billion
TFTs in a block, so that a one-terabit chip includes approximately
1,000 such blocks, if two bits of information are stored on each
TFT. Ideally, block erase is carried out by applying around 20
volts to the P- sublayer 222 (see, e.g., FIG. 2b-1) of every active
strip through connection 290 (see, e.g., FIG. 3) while holding all
global word-lines in the block at 0 volts. The duration of an erase
pulse should be such that most TFTs are erased to a slight
enhancement mode threshold voltage, between zero and one volts.
Some TFTs may overshoot and be erased into depletion mode (i.e.,
having a slightly negative threshold voltage). Such TFTs are
required to be programmed into a slight enhancement mode threshold
voltage subsequent to the termination of the erase pulses, as part
of the erase command.
Alternatively, in lieu of V.sub.bb applied to P- sublayer,
sublayers 221 and 223 on all active strips are raised to around 20
volts while holding all global word-lines at zero volt for the
duration of the erase pulse. This scheme requires that strip-select
decoders (206-0, 206-1 in FIG. 2c) employ transistors that can
withstand the 20 volts at their junctions. Alternatively, all but
the addressed global word-line are held at zero volts, while
pulsing the addressed global word-line to -20 volts and holding all
active strips in planes 202-0 through 202-7 at zero volts. This
method erases only the X-Z slice of all TFTs touched by the one
addressed global bit line.
Erase for the NROM TFT embodiment can be achieved using the
conventional NROM erase mechanism of band-to-band tunneling-induced
hot hole injection. To neutralize the charge of the trapped
electrons: apply -5 volts on the word-line, zero volts on source
sublayer 221 and 5 volts on drain sublayer 223.
The above detailed description is provided to illustrate specific
embodiments of the present invention and is not intended to be
limiting. Numerous variations and modification within the scope of
the present invention are possible. The present invention is set
forth in the accompanying claims.
* * * * *